1. Field of the Disclosure
The present disclosure relates generally to a method and apparatus for wireless communication, and more particularly, to a method and apparatus for improving communication efficiency within a basic service set (BSS) including a plurality of next-generation devices and legacy devices and within a BSS including next-generation devices which overlaps with another BSS including legacy devices.
2. Description of the Related Art
IEEE 802.11 networks generally operate using a distributed contention-based medium access. A device wanting to initiate a transmission must wait until it sees that the medium is idle for a particular length of time; that time is comprised of an inter-frame space (IFS) plus a number of backoff slots. There are a number of different IFSs used for different purposes. The most common from the IEEE 802.11 standard includes short IFS (SIFS); most of the others are formed by adding integer multiples of the Slot Time. SIFS is used between packets in a non-interruptible frame exchange sequence, e.g. a Data frame followed by its Acknowledgement.
New frame exchanges will use one of the longer IFSs. Devices supporting quality of service (QoS) will generally use the arbitrary IFS[i] (AIFS[i]) corresponding to the QoS category i of the frame to be transmitted. Devices not supporting QoS will generally use distributed IFS (DIFS). Each device will also maintain a contention window (CW) for each supported QoS category and, for the packet to be transmitted, will select a random number of backoff slots up to the current length of the Contention Window. All of the devices with something to transmit that see the medium idle for the IFS they selected will then decrement their backoff slot number for each additional Slot Time that they see the medium remains idle. If the number of backoff slots reaches zero then a device is allowed to transmit. If the medium becomes non-idle before that happens then the countdown is suspended; it is resumed from where it left off (not reset) when the medium is again idle for at least the SIPS.
There is a possibility that multiple devices will start to transmit at the same time if their backoff all end after the same number of slots. This will usually result in the frame not being acknowledged. In this case the device will increase the size of its Contention Window (up to a maximum size depending on the QoS category). This makes it less likely that the next transmission will be initiated at exactly the same time as another device.
By this means multiple devices independently contend for access to the medium. Whilst there are optional centralized scheduling or coordination mechanisms they are not commonly implemented or used. Higher priority QoS categories use shorter AIFS[i] and smaller maximum Contention Windows to give prioritized access.
All of the above is the existing behavior in the IEEE 802.11 standard and of the legacy devices currently deployed.
The forthcoming IEEE 802.11ax amendment to the standard, Enhancements for high efficiency WLAN (HEW), is intended to improve the performance in highly congested environments where the existing medium access approach does not work well. The problem is that HEW devices, e.g., next generation devices, supporting the HEW amendment will be operating in an environment where there are non-HEW devices, i.e., legacy devices, that will continue to operate using the existing medium access mechanisms. The non-HEW devices may be operating independently on the same channel, i.e. an overlapping BSS (OBSS), or a HEW access point (AP) may be supporting a mixture of HEW and non-HEW devices within the same BSS.
The HEW devices will be cooperating to improve the overall performance for devices operating within the BSS. However, the non-HEW devices will behave according to the distributed contention-based medium access method described above. Consequently, without any mitigation, this is likely to result in the non-HEW devices obtaining better (or the same) performance than the HEW devices, and preventing the new HEW mechanisms from working effectively.
Therefore, there exists a need for a method and apparatus for improving communication efficiency within a BSS including a plurality of next-generation devices and legacy devices and within a BSS including next-generation devices which overlaps with another BSS including legacy devices.